1. Field of the Invention.
The present invention pertains to the design of a hollow fiber membrane bundle to be used for selective separation of components of a fluid mixture. It is especially adaptable to the separation of the components of a mixture of gases and will be described in that context, although it should be understood that the invention per se is applicable to any fluid separation process.
In a typical gas separation a gas mixture such as air is fed at some controlled pressure to an "upstream" surface of a membrane. The membrane material acts as a selective barrier or transport medium, so that one or more of the components of the mixture move(s) through it at a higher rate than one or more other component(s).
Thus, a gas composition emerges on the "downstream" surface of the membrane (permeate) enriched in one or more components of the feed mixture, while the gas remaining on the "upstream" surface (raffinate) is depleted in same. Alternatively, it can be said that raffinate has been enriched in the component(s) less favored for permeation through the membrane.
Several chemical and physical factors influence the separation: Chemical properties and physical morphology of the membrane material, as well as the chemical and physical properties of the gases to be separated all play roles in determining how and whether a membrane will perform a useful separation. The relevance of many of the important variables may be generally summarized as follows, recognizing that the issues may be rather more complex than stated.
(1) If the membrane comprises a material having a system of very fine interconnected pores the passage of smaller gas molecules may be favored over the passage of larger ones by a form of size exclusion.
(2) In a porous material whose pore diameters are substantially smaller than the mean free path of gas molecules in Brownian motion, those of lower molecular weight will move more rapidly through the material than those of higher molecular weight in proportion to the square roots of their molecular weights. This mechanism is known as Knudsen flow separation and is illustrated in U.S. Pat. No. 4,482,360.
(3) If the membrane material is nonporous, such as a pinhole-free fully dense polymer film, different gases have different solubilities and different diffusion rates through the solid. Net flow rate through the membrane is the product of solubility and diffusivity. Different gases having different "solution-diffusion" coefficients will exhibit different net permeation rates. It is noteworthy that the absolute flow rate of gases through pores as such as in Knudsen flow is several orders of magnitude higher than the fIow rate experienced in solution-diffusion.
(4) Some of the same factors which affect the interaction of a gas molecule with those comprising a dense membrane can also be at work in a porous material where only size exclusion or Knudsen separation might have been expected. Interaction between gas molecules and those comprising the pore solid surface may be such as to induce selective enhancement of the flow of one gas component over that of another by the mechanism called "surface flow" separation.
2. Prior Art.
The membrane material may be polymeric or non-polymeric. For instance, certain metals, palladium being one, have a remarkable capacity to absorb hydrogen. Foils consisting of these metals are excellent media for selectively separating hydrogen from mixtures with other gases, as illustrated in U.S. Pat. No. 3,428,476. In respect to size exclusion and Knudsen flow, there are many forms of microporous ceramic and glass materials as well as microporous polymers which are effective.
Dense polymers are most widely employed for gas separation either as flat sheets spiral wound flat sheets or hollow fibers. The latter are most preferred.
As for fine structure or morphology of the membrane material, there are four or five distinguishable situations. The simplest one is that in which the membrane is simply a fully dense single polymer in the form of a sheet or hollow fiber. Many polymers have been applied this way as in U.S. Pat. Nos. 3,415,038, 3,335,545, 3,798,185, and 4,132,824 among many others.
Another morphology is that of the asymmetric membrane which is comprised of a single polymer that has been formed into a sheet or fiber having a relatively thin fully dense surface supported by a much thicker highly porous region. In hollow fibers the skin is most often on the outside of the fiber, but it is not unusual for it to be on the inside. U.S. Pat. Nos. 3,674,628, 4,127,625, 4,239,793, and 4,472,175 are exemplary.
A variant of asymmetric membrane is the so-called "occlusively coated" membrane. Here, an imperfect ultrathin barrier layer skin is treated with a solution of a highly permeable low-selectivity material which plugs surface flaws and pinholes in such a way that the selective properties of the parent polymer skin can be expressed. U.S. Pat. Nos. 4,230,463 is illustrative of this situation.
Another useful structure for gas separation membranes is the so-called "composite" wherein a thin fully dense film of selectively permeable polymer is formed on a porous substrate, comprising a different polymer. U.S. Pat. Nos. 3,616,607, 3,980,456, and 4,826,599, are illustrative. The substrate is not responsible for the selective permeation of a gas mixture, but rather acts as an inert support for the ultrathin permselective skin. In hollow fibers, the dense film is almost always formed on the outside surface of the substrate.
It has now also been found that a permselective layer can be formed as a fully dense region at neither the outer nor inner surface of the fiber wall, but as a zone within the pore system comprising the wall of a hollow fiber. U.S. Pat. No. 4,838,904 is illustrative of this situation.
Still another method of conferring gas selectivity on a non-selective porous substrate hollow fiber has been described in U.S. Pat. No. 4,784,880. Here a bundle of fibers having a microporous barrier layer surface surmounting a porous main body is treated with a solution admitted into the bores while evaporation is promoted at the barrier layer surface so that the barrier layer is densified or filled with solute.
When hollow fibers are used for gas separation, pressurized feed may be applied either to their external surfaces or to their bores. When the former mode is used, the fibers are assembled as a compact bundle installed in a pressure-tight containment vessel, ("shell"). The bundle and shell comprise a "module". Many such have been described but U.S. Pat. No. 4,315,819 is illustrative here.
When the pressurized feed gas is admitted into the module to contact the outer surfaces of the fibers the mode is known as shell-side feed to distinguish from the case where pressurized gas might be delivered into the fiber bores, which would be called bore-feed, as in U.S. Pat. Nos. 4,734,106 and 4,881,955. In the bore-feed mode bundle geometry is not a particularly consequential factor in determining good fluid dynamics of the overall process. In shell-side feed uniform controlled flow of feed gas over the outer surfaces of the fibers is critical.
Fiber bundles have been assembled in a number of geometries which may be roughly divided into two types. In one such type, a number of cut lengths of fiber are gathered together into an essentially straight parallel array. This is illustrated in U.S. Pat. Nos. 4,315,819 and 4,929,259. The other principal bundle style involves assembling initially uncut lengths of fiber by winding them on a frame or mandrel which rotates while the fibers are caused to reciprocate repetitively from end to end of the bundle. In so doing layers of fiber wraps are built up wherein fiber paths at each end of the bundle form re-entrant loops. A prevailing way of accomplishing this involves winding overlapping helices one upon another as in U.S. Pat. Nos. 4,430,219 and 4,631,128.
Whether the bundle is formed from cut lengths or wound in some fashion from continuous uncut lengths, at least one end of the bundle is potted into a polymer plug or thick sheet and the fibers and plug are sliced or perforated in some way to open all the fiber bores to permit exit of fluid permeated to the bore. The potting plug makes a gas-tight seal to the containment shell. U.S. Pat. Nos. 4,061,574 and 4,207,192 are exemplary.
Quite often both ends of the bundle are potted. When this is done there is the option of opening fiber bores at one end or both ends and there are different reasons for doing each. When a bundle has been assembled from cut lengths of fiber it is almost invariably potted at both ends. Either both plugs are sliced to open the bores at each end, or one end may be left unsliced in order to keep sealed the initially open fiber ends embedded in that plug.
When the bundle has been assembled from continuous uncut lengths which follow re-entrant paths at each bundle end, potting of the uncut end is sometimes used as an expedient to aid in bundle handling. This is illustrated in U.S. Pat. No. 4,781,834. However, unlike bundles assembled from cut lengths, a bundle having re-entrant loops of fiber at each end does not require potting both ends in order to seal off already cut ends when it is desired to have only one end of the bores open.
For either bundle style, however, end-use operational considerations ma dictate that both ends be potted and sliced because this permits permeate to exit through twice as many outlets and the bore length to be traversed by permeate is halved. Both effects may improve flow dynamics in some cases. U.S. Pat. Nos. 3,442,002 and 4,451,369 represent widely different manifestations of this arrangement.
The two-ends-open design offers another functional option. One end may be used for the introduction of sweep gas into the bore while the other end provides the outlet mean for both permeate and sweep gas. "Sweeping" tends to influence transmembrane gas composition equilibria in such a way as to improve net separation efficiency. The use of sweep gas may be desirable whether the bundle is an array of straight parallel fibers or an assembly of helically wound fibers or any other configuration.
In the cases where it is desired to have fiber bores opened only at one potted bundle end, there are two possible situations. As has already been stated, when cut lengths are assembled into a bundle open ends of fiber need to be sealed off at one end and embedding this end in potting is a convenient way to do this while also stabilizing the bundle configuration. In the case where the bundle is assembled from continuous fiber lengths with re-entrant loops at each bundle end, there is no need and there may well be a disadvantage to potting more than the one end at which bores are to be opened. U.S. Pat. No. 3,660,281 is exemplary.
Whatever the bundle style or the preferred potting and slicing alternative, when feed gas is admitted under pressure into the module and contacts the outer surfaces of the fibers one or more component(s) of the feed gas permeates from that surface across the fiber wall to its bore. Non-permeated gas is allowed to exit the module and constitutes the raffinate. The admission of feed may be via a perforated tube lying in the central axis of the bundle and the flow of gas is radially outward toward the inner surface of the containment vessel. This is illustrated in U.S. Pat. No. 3,422,008.
However, a preferred arrangement provides that feed enters the bundle at one end where it is induced to disperse radially before following a path more or less parallel to the fiber axes. Permeation occurs and the residual gas becomes a raffinate which is allowed to exit the bundle at the end opposite the feed region. This is illustrated in U.S. Pat. No. 4,781,834.
Contents of the bore must of necessity flow parallel to the fiber axis toward an open end of the bore. In the situation where feed flow is directed radially within the bundle the process is described as being operated in the cross-flow mode. Such a situation requires that there be a feed distributer or raffinate collector tube at the center of the bundle. This is not the case where feed flow is in a direction parallel to the bundle axis. This bundle may have a support element in the bundle axis or not. In parallel flow there are two options: 1) the case where permeate flow in the bore and shell side feed flow are in the same direction, known as parallel coflow, and 2) the case where the two flows are in opposite direction, known as parallel counterflow. For most uses parallel counterflow is considered the superior option.
Very few practical membrane separations are operated with only a single module. Rather, an installation may have from at least two up to several hundred modules linked together by plumbing which directs flows from one group of modules to another in a number of steps. The system is in effect organized into separate but interconnected zones, an arrangement described as a cascade.
Whether the system is so arranged or not it frequently is the case that several bundles are housed in a common pressure shell. In U.S. Pat. No. 4,632,756 a group of bundles receives a common feed in parallel and therefore such a group comprises a single zone. In U.S. Pat. Nos. 4,451,369 and 4,508,548, however, bundles are arranged so that the raffinate from a first bundle is the feed for a second bundle. In U.S. Pat. No. 4,508,548 the pressurized gas flows in the fiber bores, in U.S. Pat. No. 4,451,369 the feed and raffinate are on the shell side. In either instance, however, the effect of the multi-bundle arrangement is to provide two or more zones inside a common containment vessel (i.e. pressure shell).
The multi-zone principle has been extended to the design of individual bundles. In U.S. Pat. No. 4,676,808 fibers are wound on a mandrel and layers are built up. After a sufficient thickness of wraps has been accumulated a thin gas-impermeable film is applied over the first thickness of wraps and fiber winding continued until the desired total amount of fiber has been assembled. Ultimately, potting and slicing provides that the fibers within the bundle are actually segregated into zones which are separated by the film.
In U.S. Pat. No. 4,220,535 a different kind of zoning is created within a single bundle. A barrier is formed transverse to the bundle axis closer to one of its ends than the other. The bundle is potted at both ends and fiber bores opened at both bundle ends. Separate perforated tubes lie in the central axis of the bundle, each traversing from a closed end adjacent the barrier through a bundle zone and then through one or the other potting plug. Thus each bundle zone is accessible to the flow of fluid into or out of its own central tube.
The barrier in U.S. Pat. No. 4,220,535 acts essentially as a flow director for unimpeded fluid communication between the active fiber surfaces in each of the two zones. Being a two-ends-open configuration the fibers are susceptible to having a sweep fluid propelled down their bores from one end to the other. Feed fluid enters the module via one central tube through a first end-plug and flows radially over the active surfaces of fibers in a first zone. After moving through the bundle to the shell surface in the first zone, the residue of the feed then passes between the shell and the barrier into the second zone and flows radially toward the second perforated central tube for passage from the bundle through the second potting plug. Thus, although there are two fiber zones the active surfaces of the fibers in each zone are in direct fluid communication.
Barriers introduced across the bundle in regions other than the ends have also been employed for other reasons than to create zones with different feed qualities. Bundles made from cut fiber lengths are notorious for having regions of varying fiber packing density resulting in a tendency for feed to flow irregularly within the bundle. Transverse baffle elements might be expected to prevent flow bypassing some regions and favoring others. U.S. Pat. No. 4,367,139, therefore, teaches partial barriers disposed transversely to the bundle axis at selected locations in order to induce uniform flow among the fibers in the parallel flow mode. All active fiber surfaces are, however, in fluid communication.
3. Summary of Prior Art
Although some variables in bundle design are interactive so that choosing an option for one variable limits choice in another, it is still useful to summarize the foregoing as follows:
______________________________________ Factor Option ______________________________________ separation mech. size exc/ Knudsen/solution-diff/surface flow fiber type dense/asymmetric/composite/occlusive coated selective surf. inside outside bundle style straight cut length uncut re-entrant loop potting both ends one end only bore open at both ends one end only feed to the bore shell-side flow in bundle radial axial bore/shell flow coflow counterflow zoning radial axial ______________________________________